One form of free-standing glass fibre antenna is shown in U.S. Pat. No. 3,725,944.
The antenna disclosed there is formed of multiple layers of glass fibres laid up in a certain manner, and incorporates a plurality of radiating members extending in a generally longitudinal linear fashion up the antenna.
Antennas made in accordance with such patent, were found to be greatly superior to other free-standing antennas which had hitherto been available, and were able to withstand stresses due to weather, wind and the like, to a greater extent than any previous antennas then available.
Such antennas are particularly useful in military applications, in particular in mobile military applications such as at sea, or in situations where a powerful long-distance antenna must be set up and in operation at very short notice.
When they are subject to repeated bending stresses, for example due to high winds, or due to violent movement of the base upon which they are mounted, i.e., a ship in a rough seal, substantial stresses are imposed in the radiating conductors. Accordingly, a plurality of such conductors, all of them being generally linear members are employed. In this way, even through some such conductors would gradually break down, under repeated flexing of the antenna, the antenna would continue to function.
However, the use of a plurality of radiating members, of a generally linear nature, arranged spaced apart radially around the structure of the antenna imposes certain limitations on the effectiveness of the antenna for radiating radio transmissions.
It is of course well known that an antenna must be tuned to the resonant frequency of the transmission. This may be done by varying the length of the antenna, in simple cases. However, in the present invention, the length of the antenna is determined initially in the design stage and once erected, it cannot be changed. Tuning of the antenna is therefore usually effected by the use of a coil, connected at the base of the antenna. In the present case, where very high powered transmissions are involved, this introduces further problems and limitations, and also adversely affects the radiation characteristics of the antenna itself.